Multi-level reticle system and method for forming multi-level resist profiles

ABSTRACT

A method is providing for making a multi-level reticle which transmits a plurality of incident light intensities, which in turn, are used to form a plurality of thicknesses in a photoresist profile. A partially transmitting film, used as one of the layers of the reticle, is able to provide an intermediate intensity light. The intermediate intensity light has an intensity approximately midway between the intensity of the unattenuated light passing through the reticle substrate layer, and the totally attenuated light blocked by an opaque layer of the reticle. The exposed photoresist receives light at two intensities to form a via hole in the resist in response to the higher intensity light, and a connecting line to the via at an intermediate level of the photoresist in response to the intermediate light intensity. A method for forming the multi-level resist profile from the multi-level reticle is provided as well as a multi-level reticle apparatus.

This application is a divisional application of Ser. No. 08/660,870,filed Jun. 10, 1996, entitled MULTI-LEVEL RETICLE SYSTEM AND METHOD FORFORMING MULTI-LEVEL RESIST PROFILES, and invented by Nguyen et al.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates generally to integrated circuit processes andfabrication, and more particularly, to a multi-level reticle andfabrication method for producing multi-level photoresist patterns.

The demand for progressively smaller and more powerful electronicproducts, in turn, fuels the need for smaller geometry integratedcircuits (ICs), and large substrates. It also creates a demand for adenser packaging of circuits onto IC substrates. The desire for smallergeometry IC circuits requires that the interconnections betweencomponents and dielectric layers be as small as possible. Therefore,research continues into reducing the width of via interconnects andconnecting lines. Copper is a natural choice to replace aluminum in theeffort to reduce the size of lines and vias in an electrical circuit.The conductivity of copper is approximately twice that of aluminum andover three times that of tungsten. As a result, the same current can becarried through a copper line having half the width of an aluminum line.

The electromigration characteristics of copper are also much superior tothose of aluminum. Copper is approximately ten times better thanaluminum with respect to electromigration. As a result, a copper line,even one having a much smaller cross-section than aluminum line, is ableto maintain electrical and mechanical integrity.

There have been problems associated with the use of copper, however, inIC processing. Copper pollutes many of the materials used in ICprocesses and, therefore, care must be taken to keep copper frommigrating. In addition, copper is especially prone to oxidation,especially during oxygen etch processes. Care must be take to protectcopper from exposure during etch processes, annealing, and processesrequiring elevated temperatures. Also, the oxidation products of copperare difficult to clean. In addition, copper cannot be deposited ontosubstrates using the conventional processes for the deposition ofaluminum when the geometries are small. That is, new depositionprocesses have been developed for use with copper, instead of aluminum,in the lines and interconnects of an IC interlevel dielectric.

It is impractical to sputter metal, either aluminum or copper to fillsmall diameter vias, the gap filling capability is poor. To depositcopper, a chemical vapor deposition (CVD) technique has been developedin the industry. However, even with the CVD technique, the conventionetch process cannot be used. The low volatility of copper etch productsrequire copper to be removed (vaporized) at high temperatures,approximately 250° C., which is too high for photoresist masks. Wetetches are isotropic, and so too imprecise for many applications.Therefore, the IC processing industry has developed a process to form avia using CVD without etching the copper. The new method is called theinlay, or damascene, process.

The damascene method for forming a via or interconnect between asubstrate surface and an overlying dielectric surface is describedbelow. The underlying substrate surface is first completely covered witha dielectric, such as oxide. A patterned photoresist profile is thenformed over the oxide. The resist profile has an opening, or hole, inthe photoresist corresponding to the area in the oxide where the via isto be formed. Other areas of the oxide to be left in place are coveredwith photoresist. The photoresist covered dielectric is then etched toremove oxide underlying the hole in the photoresist. The photoresist isthen stripped away. CVD copper is then used to fill the via. A layerconsisting of oxide with a copper via through it now overlies thesubstrate surface. The excess copper remaining is removed with achemical mechanical polish (CMP) process, as is well known in the art.

Since the damascene processing method is relatively new to the ICindustry, refinements in the technique are ongoing. One refinement isthe dual damascene method. In the dual damascene method vias,interconnects, and lines are formed in a dielectric at two differentlevels. In terms of the example of the damascene process in thepreceding paragraph, the dual damascene process adds a second via, orinterconnecting line, in the deposited oxide that extends from the new(oxide) surface to a level in the oxide between the underlying substratesurface and the new (oxide) surface. The dual damascene method isdescribed in greater detail in FIGS. 1-6 as prior art in co-pendingpatent application Ser. No. 08/665,014 filed Jun. 10, 1996, entitled"Method for Transferring a Multi-level Photoresist Pattern", invented byTue Nguyen, Sheng Teng Hsu, Jer-shen Maa, and Bruce Dale Ulrich, DocketNo. SMT 162 which is assigned to the same assignees as the instantpatent.

One known method of performing the dual damascene process is throughmultiple photoresist mask and etch steps. A single level photoresistprofile is formed on a layer deposited dielectric and a via pattern isformed by etching to a first interlevel in the dielectric material. Atthis point in the process the via is only partially etched. Thephotoresist is then stripped and a second single layer photoresistprofile is formed on the dielectric surface to form an interconnectpattern to a second interlevel in the dielectric material. Theinterconnect is formed by etching. Coincident with etching theinterconnect, the via is etched such that interconnects in underlyingsubstrate layers are exposed to allow electrical contact. Aligning thephotoresist profiles is a problem using this method. If the twophotoresist profiles are not aligned correctly, then intersectingfeatures in the dielectric material will be misaligned. That is, aconductive line associated with the first photoresist pattern may notcorrectly intersect a via associated with the second photoresistprofile. Alignment errors can be corrected by making the intersectingfeatures oversized, but this takes away from the overall goal ofreducing the size of connecting lines and vias. Alignment problemsreduce yields, and increase cost and the complexity of IC processes.

Another known method of performing the dual damascene process usesphotoresist profiles having multiple levels, or thicknesses, to formvias and interconnect at multiple levels in an IC dielectric. Anelectron beam or laser may be used to directly write a multi-levelpattern into photoresist, but is not commercially practical. So called"gray-tone" masks, formed from repetitive patterns of dots that appearas transparent holes on the chromium mask of the reticle, have also beenused to form multi-level resist profiles as described by Pierre Sixt,"Phase Masks and Gray-Tone Masks", Semiconductor FabTech, 1995, page209. Sixt also gives a general description for a process to transfer themulti-level resist onto a dielectric. The process relies on a one-to-oneetch selectivity between the dielectric material and the resistmaterial. The dielectric and the overlying photoresist profile are thenetched together so that any exposed dielectric material is etched at thesame rate as overlying photoresist material. Thinner layers of resistcause a deeper etch into the dielectric so that, after etching, thedielectric shape generally resembles the photoresist pattern overlyingthe dielectric at the beginning of the process. One problem with thismethod is finding dielectric materials and photoresist materials thathave identical etch selectivity. It is also difficult to transfervarious features, especially small or relatively complicated features,into a dielectric using this method. Polymers and by-products of theetch process tend to collect on areas of the resist pattern, changingthe shape and etch rates of the resist profile. Further, the articlediscloses that vias made by this method have a relatively large size,approximately 25 μm, due to the resolution limits imposed with the pixelsize in the gray-tone mask. Vias of this size are approximately twoorders of magnitude larger than vias formed through conventionalmethods, and are unsuited for most IC processes.

It is well known in the art to use a multi-level reticle for the phaseshifting of light in the production of photoresist masks. Thesemulti-level, or phase shifting, reticles are used to reduce theconstructive, and unintended, interference of light patterns diffractingfrom a reticle aperture. Constructive interference is the in-phaseaddition of light from two different sources. The incidence of light,even a highly coherent light, upon an aperture produces at least somediffraction. The pattern of light diffracted through an aperture isdependent upon the aperture shape and wavelength of light as is wellknown in the art.

A conventional, or bi-level reticle, is composed of a translucentsubstrate which transmits essentially all incident light, and an opaquesubstrate which transmits substantially no incident light. Lighttransmitted through a square aperture bi-level reticle produces thegeneral aperture shape with some diffraction of light around the edgesof the aperture shape. One general problem with constructiveinterference occurs in the area between two vias to be formed on aphotoresist mask from light passing through the two correspondingaperture holes in the reticle. Constructive interference of lighttransmitted through the two via apertures often occurs between the twovias, causing an unintended area of thin resist which ultimatelytranslates into an imperfection in the IC substrate etched from thephotoresist mask. This process is explained in greater detail in FIG. 1in the Detailed Description of the Preferred Embodiment of thisinvention.

Phase shifting reticles were developed to minimize constructiveinterference problems, as mentioned above. The general principle ofphase shifting reticles is to change the phase of light to promotedestructive interference in the areas of the photoresist mask subject tomultiple diffraction sources. That is, the light from one diffractionsource is adjusted to have a phase difference of 180° from the seconddiffraction source so that the diffraction effect of the two sources areself canceling.

A typical method of performing this 180° phase shift is to use so called"half tone", or partially transmitting, films. A typical phase shiftingreticle, using a half tone film, is disclosed in U.S. Pat. No. 5,358,827by Garofalo, et al. Other phase shifting multi-level reticles aredisclosed by Kobayashi, Oka, Watanabe, Inoue and Sakiyama in "TheControl of Sidelobe Intensity of the Chrome Pattern (COSAC) in Half-TonePhase shifting Mask", Extended Abstracts of the 1995 InternationalConference on Solid State Devices and Materials, Aug. 21 through 24,1995, pp. 935-937. Another source describing a similar phase shiftingreticle is by Levenson, Viswanathan and Simpson, "Improving Resolutionin Photolithography with a Phase shifting Mask", IEEE Transactions onElectron Devices, Vol. ED-29, No. 12, December, 1982.

The above disclosures reveal a reticle constructed of a transparentsubstrate made of a quartz material to transmit substantially allincident light. The reticle is constructed with a half-tone, or phaseshifting film over the substrate to shift the phase of transmittedlight. Over the half-tone layer is an opaque film to substantially blocktransmitted light. Through the use of phase shifting, to producedestructive interference, these reticles produce light at substantiallytwo intensities, 100% intensity and 0% intensity, to form a single-levelphotoresist mask as is well known in the art. Alternately, it can besaid that the reticle produces light at a single intensity (100%transmission), and otherwise blocks (0% transmission) the light. Phaseshifting performed with the single level photoresist mask is for thepurpose of more clearly defining features, such as vias, and to reducethe effects of diffraction. Typically, conventional half-tone materialis chosen with regard to its phase shifting characteristics, as opposedto its light attenuation characteristics. Therefore, the half-tone filmsin the phase shifting reticles are chosen to phase shift transmittedlight 180° while providing substantially no attenuation, as disclosed inthe Levenson article. Alternately, half tone films are chosen to phaseshift transmitted light 180° while substantially attenuating theintensity of transmitted light as disclosed in Kobayashi, et al.

It would be advantageous to utilize the intensity attenuationcharacteristics of half-tone films in the production of photoresistmasks.

It would also be advantageous to use the intensity attenuationcharacteristics of half-tone films to make photoresist masks, orpatterns, having multi-levels to perform etching into IC substratematerial to a plurality of depths.

It would be advantageous to combine the light intensity attenuationcharacteristics of a half-tone film to create photoresist masks with aplurality of thicknesses, with phase shifting characteristics of ahalf-tone film to create sharp features and to reduce errors caused bydiffraction.

Accordingly, a reticle is provided through which incident light ispassed to define predetermined areas of illumination on a lightsensitive photoresist surface. The reticle comprises a firsttransmission level film producing transmitted light of a firstintensity, a second transmission level film producing transmitted lightof a second intensity greater than the first intensity, and a thirdtransmission level film producing transmitted light of a third intensitygreater than the second intensity.

Second transmission level film transmits more than approximately 10%,but less than approximately 90%, of incident light, whereby theattenuation characteristics of the second transmission level film areapproximately mid-way between the first and third transmission levelfilm attenuation characteristics, such that the reticle, when directedto a light sensitive surface, forms at least three distinctiveintensities on the illuminated areas of photoresist.

A method is also provided for using photolithography to form a reticleon a reticle substrate, to transmit incident light. The method comprisesthe steps of depositing at least one film, to partially transmitincident light, over the reticle substrate, the partially transmittingfilm diminishing the intensity of light at predetermined percentage intransmission through the partially transmitting film, and the substratepassing substantially all light incident to the substrate. A methodcomprises the step of depositing an opaque film over the reticlesubstrate the opaque film blocking light so that substantially allincident light is attenuated. A method also comprises the step ofetching selective portions of the opaque film deposited earlier, and thepartially transmitted film deposited earlier, to reveal predeterminedareas of reticle substrate and partially transmitting film, wherebylight introduced to the reticle is transmitted through the predeterminedareas of reticle substrate, partially transmitting film, and remainingopaque film to produce at least three intensities of light.

Further, a method is provided for forming a photoresist profile on asubstrate comprising the steps of providing a layer of photoresisthaving a predetermined thickness on the substrate, and directing lightto the photoresist through a reticle having a first transmittingintensity to create a first exposure pattern in the photoresist, and thereticle having a second transmitting intensity to create a secondexposure pattern in the photoresist. The method also including the stepof developing the photoresist to remove a first thickness ofphotoresist, less than said predetermined thickness, in the areas ofsaid first exposure pattern, and to remove a second thickness of thephotoresist in the areas of the second exposure pattern, whereby theprofile includes areas of photoresist having a plurality of differentthicknesses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view of a reticle to form threevias, and the relative intensities of light transmitted through thereticle (prior art).

FIG. 2 is a partial cross-sectional view of a tri-level reticle with anoverlying first photoresist pattern.

FIG. 3 is a partial cross-sectional view of a tri-level reticle of FIG.2 after the first etch step.

FIG. 4 is a partial cross-sectional view of the tri-level reticle ofFIG. 3 with an overlying second photoresist pattern.

FIG. 5 is a partial cross-sectional view of the tri-level reticle ofFIG. 4 following the second etch step.

FIG. 6 is a partial cross sectional view of a bi-level photoresistpattern formed from a tri-level reticle.

FIG. 7 is a partial cross sectional view of a bi-level photoresistpattern formed from a tri-level reticle using phase shifting to enhancethe resolution of the pattern.

FIG. 8 is a graph illustrating the relationship between resistthickness, exposure time, and the transmission characteristics of apartially transmitting film.

FIG. 9 is a partial cross-sectional view of a multi-level reticle.

FIG. 10 is a partial cross-sectional view of the multi-level reticle ofFIG. 9 after the first etch step.

FIG. 11 is a partial cross-sectional view of the multi-level reticle ofFIG. 10 with an overlying second photoresist pattern.

FIG. 12 is a partial cross-sectional view of the multi-level reticle ofFIG. 11 after the second etch step.

FIG. 12a is a partial cross-sectional view of a multi-level reticlehaving a plurality of partially transmitting level films.

FIG. 13 is a partial cross-sectional view of a tri-level reticletransmitting incident light at three intensities on a light sensitivesurface.

FIG. 14 illustrates the steps in the method of the present invention toform a tri-level reticle.

FIGS. 15(a) and (b) illustrate the steps of the method of the presentinvention to form a multi-level photoresist pattern.

FIG. 16 illustrates the steps of the method of the present invention topattern a photoresist film with a plurality of thickness from a singleexposure to light.

FIG. 17 illustrates the method of the present invention to form aphotoresist profile having two exposure patterns from a single reticle.

FIGS. 18(a) through 18(e) illustrate an alternate embodiment of thepresent invention, a method of forming a multi-level reticle from amulti-level photoresist profile.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a partial cross-section view of a reticle used to form threevias, and the relative intensities of light transmitted through thereticle. Incident light 10, from a light source not shown, is introducedto a reticle 12. Reticle 12 comprises a reticle substrate 14 made from amaterial such as quartz. Overlying substrate 14 are two areas of a phaseshifting film 16 and 17. Overlying phase shifting film 16 and 17 are twoareas of an opaque film 18 and 19, such as chrome. Transmitted light 20exits reticle 12 to a photoresist film, not shown. A graph 22 of therelative light intensities is displayed. Where incident light passesthrough substrate 14, the transmitted light is substantially at a 100%intensity relative to the incident light. In areas where the incidentlight is introduced to phase shifting film 16 and opaque film 18, thetransmitted light is substantially zero. The diffraction of lightpassing through a first aperture 24 and a second aperture 26 ismitigated by the effect of light transmitted through phase shifting film16. Without phase shifting film 16, the diffraction through apertures 24and 26 constructively interferes to form the higher intensity profile 30indicated by the dotted lines in graph 22. Likewise, if phase shiftingfilm 17 is removed, a higher intensity, caused by constructiveinterference from aperture 26 and aperture 32, is formed indicated bydotted line 34. The areas of higher, unintended, intensity indicated bydotted lines 30 and 34 yield deformities in a photoresist profile 36formed from reticle 12. The deformities in photoresist 36 indicated bydotted lines 38 and 40 will ultimately yield unintended etching in an ICsubstrate etched from photoresist profile 36.

FIGS. 2-5 illustrate the formation of a tri-level reticle using themethod of the present invention. FIG. 2 is a partial cross-sectionalview of a tri-level reticle with an overlying first photoresist pattern.Reticle 50 comprises a quartz substrate 52, a partially transmittingfilm 54, an opaque film 56, and overlying first photoresist pattern 58.

Photoresist 58 is patterned with an opening 59 for etching into reticle50. FIG. 3 is a partial cross-sectional view of tri-level reticle 50 ofFIG. 2 after the first etch step. An opening etched through opaque film56 and partially transmitting film 54 to exposes a predetermined area 60of quartz substrate 52. The opening underlies opening 59 in resist 58.

FIG. 4 is a partial cross-sectional view of tri-level reticle 50 of FIG.3 with an overlying second photoresist pattern 62. Second resist pattern62 has an opening to reveal a predetermined area 63 of opaque film 56.

FIG. 5 is a partial cross-sectional view of tri-level reticle 50 of FIG.4 following the second etch. An opening etched through area 63 of opaquefilm 56 exposes a predetermined surface area 64 of partiallytransmitting film 54.

FIG. 5 describes reticle 50 through which incident light is passed todefine predetermined areas of illumination on a light sensitivephotoresist surface, reticle 50 comprises a first transmission film 56producing transmitted light of a first intensity, a second transmissionlevel form 54 producing transmitted light of a second intensity greaterthan the first intensity, and a third transmission level film 52producing transmitted light of a third intensity greater than the secondintensity. In the preferred embodiment of the invention thirdtransmission level 52 is a substrate, second transmission level 54overlies third transmission level substrate 52, and first transmissionlevel film 56 overlies the second transmission level film 54.

It is a feature of the present invention that first transmission levelfilm 56 is an opaque film selected from a group consisting of chromium(Cr), chromium oxide (CrO), and iron oxide, whereby first transmissionlevel film 56 substantially blocks incident light. It is also a featureof the invention that third transmission level 52 is selected from thegroup consisting of quartz, synthetic quartz, and glass, whereby thirdtransmission level film 52 is transparent to pass substantially allincident light. In the preferred form of the invention firsttransmission level film 56 has a first transmission level opening,through first transmission level film 56, to reveal a predetermined area64 of second transmission level film 54, and second transmission levelarea 64 has a second transmission level opening, through secondtransmission level 54, to reveal a predetermined area 60 of thirdtransmission level film 52. Typically, the thickness of the chromiumopaque layer is 110 nanometers (nm), the partially transmitting film 80nm, and the quartz substrate 0.09 inches.

Second transmission level film 54 retards the phase of light in apredetermined number of degrees whereby the phase difference betweenreticle transmission level films improves the resolution in transmittedlight intensities to reduce constructive interference between adjacentlyilluminated areas of photoresist. It is a feature of the invention thatsecond transmission level film 54 retards the phase of transmitted lightapproximately 90°.

FIG. 6 is a partial cross-sectional view of a bi-level photoresistpattern formed from a tri-level reticle. The photoresist thickness,before exposure to light through a reticle, is approximately 2 microns,shown in the Fig. between the 11.0 and 12.8 microns demarcations. Afterexposure to light through a reticle, the bi-level resist pattern in FIG.6 is developed. Approximately 1 micron of resist has been removed toform an intermediate level having a thickness of approximately 0.5microns. To decrease the size of the via, phase shifting techniques,well known in the art to produce single-level resist profiles, are used.FIG. 7 is a partial cross-sectional view of a bi-level photoresistpattern formed from a tri-level reticle with phase shifting to enhancethe resolution of the pattern. The via is substantially smaller than thevia of FIG. 6, and intermediate surface surrounding the via is leveler.Thus the reticle uses a partially transmitting film to create anintermediate intensity to form an intermediate resist level, and thereticle uses the partially transmitting film for phase shifting tocontrol resolution. The use of 180° phase shifting film has been foundto cause excess cancellation in the area of the resist surrounding thevia. Optimum results are obtained when using only partial cancellationof diffracted light. A partially transmitting film having a phase shiftof 90°, and a transmission intensity of 30% (attenuating 70% of incidentlight) is found to be effective.

In the preferred form of the invention second transmission level, orpartially transmitting, film 54 is selected from the group consisting ofindium tin oxide and molybdenum silicon oxynitride, whereby secondtransmission level film 54 attenuates the intensity of transmittedlight, a predetermined percentage.

In the preferred form of the invention second transmission level film 54transmits more than approximately 10%, but less than approximately 90%,of incident light, whereby the attenuating characteristics of secondtransmission level 54 are approximately mid-way between firsttransmission level film 56 and third transmission level film 52attenuation characteristics, such that reticle 50, when directed to alight sensitive surface, forms at least three distinctive intensities onthe illuminated areas of photoresist.

It is a a feature of the invention that the incident light introduced toreticle 50 has a predetermined wavelength and a predetermined intensity.I-line light, generated from a mercury arc lamp, having a wavelength of0.365 μm, is typically used as the incident light source, not shown.Alternately, various other wavelengths may be used such as a kryptonfluoride laser having a wavelength of 0.248 μm. The intensity of lightupon the reticle is typically from 500 to 1000 watts. The method of thepresent invention is not limited to a special band of light wavelengths,but is applicable to all suitable wavelengths.

FIG. 8 is a graph illustrating the relationship between resistthickness, exposure time, and the transmission characteristics of apartially transmitting film. A normalized exposure of 1.0 corresponds tothe exposure time required to completely remove the photoresist (afterdevelopment.) Typically, it requires approximately 100 to 300 ms tocompletely expose the photoresist. Note that the use of a partiallytransmitting film (mask) of 30% transmission allows the use of anormalized exposure of approximately 0.5 to yield a resist thickness ofabout 10,000 Angstroms. The intermediate level thickness in thephotoresist can be adjusted by changing the dosage of light on theresist, with dosage being a function of light intensity and time.However, low dosages (much less than 0.5 normalized exposure) do notprovide enough light to consistently form a via through the resistprofile. High dosages of light (much more than 0.5 normalized exposure)often cause the vias to be over-sized, and the vias walls to becomesloped. Therefore, the use of a 30% transmission film allows an exposuretime to be selected to correctly dose the resist so that theintermediate level is formed at the proper thickness in the profile,without incorrectly dosing the area of the resist where the via isformed to detrimentally effect the formation of the via through theprofile.

FIGS. 9 through 12 illustrate the method of forming a multilevel reticlewith a plurality of second transmission level films. FIG. 9 is a partialcross-sectional view of a multi-level reticle 70. Reticle 70 comprisesan opaque film, or first transmission level film 72, a first partiallytransmitting film 74, a second partially transmitting film 76, atransparent substrate, or third transmission level film 78, and anoverlying first photoresist profile 80 with an opening.

FIG. 10 is a partial cross-sectional view of multi-level reticle 70 ofFIG. 9 after the first etch step. An opening has been etched throughopaque film 72 second partially transmitting film 76 and first partiallytransmitting film 74 to reveal a predetermined surface area 82 ofsubstrate 78. The etched opening underlies the opening in the pattern ofresist 80 shown in FIG. 9.

FIG. 11 is a partial cross sectional view of multi-level reticle 70 ofFIG. 10 with an overlying second photoresist profile 84. Resist profile84 has an opening to reveal predetermined surface areas 86 of opaquefilm 72.

FIG. 12 is a partial cross-sectional view of multi-level reticle 70 ofFIG. 11 after the second etch step. After etching, a predeterminedsurface area 88 of first partially transmitting film 74 is revealed.Light incident to reticle 70 is transmitted through opaque film 72 at afirst transmission level. Incident light is transmitted through firstpartially transmitting film 74 and second partially transmitting film 76at a second intensity. Incident light is transmitted through substrate78 in the area of exposed surface 82 at a third intensity.

Reticle 70 of FIGS. 9 through 12 is as reticle 50 of FIGS. 2 through 5,in which a plurality of second transmission level films are provided.Each of the plurality of second transmission level films 74 and 76producing transmitted light at one of a plurality of second intensitiesgreater than the first intensity and less than the third intensity,whereby light in a range of second intensities is provided. Theinvention is not limited to only two second transmission levels films,more may be used. Alternately, reticle 70 is as reticle 50 with thefirst transmission level film made up of a plurality of secondtransmission level films 74, 76, and 72, each one of the plurality ofsecond transmission levels films 74, 76, and 72 being a partiallytransmitted film layer, such that the intensity characteristics of lighttransmitted through the plurality of second level films 74, 76, and 72are cumulative to yield the light transmission characteristics of thefirst transmission level film.

That is, second transmission level films may be combined, as shown inFIG. 12, to get a transmission attenuation and phase shift that is acombination of two or more partially transmitting films. Alternately,FIG. 12a is a partial cross-sectional view of a multi-level reticlehaving a plurality of partially transmitting level films. A thirdphotoresist pattern can be formed to reveal a portion of firsttransmission level film 74 which is in turn etched to reveal an area 89of second transmission level film 76. Light is then transmitted throughthe reticle at four intensities (and phases); approximately 0% throughthe opaque layer, through area 88 having the combination of first andsecond partially transmitting films 74 and 76, through area 89 which isjust second transmission level film 76, and through area 82 of quartzsubstrate 78. This process can be repeated with more added layers ofpartially transmitting films.

It is a feature of the invention that the first transmission level filmincludes a first number, that is substantially all, of the multiplepartially transmitting film layers 74, 76, and 72, and the secondtransmission level film includes a second number, fewer than the firstnumber, of the partially transmitting film layers 74, 76, and 72. Theinvention is not limited to a first transmission level film of onlythree partially transmitting film layer, more may be used.

FIG. 13 is a partial cross-sectional view of a tri-level reticletransmitting incident light at three intensities on a light sensitivesurface. Tri-level reticle 90 comprises a transparent substrate 92, apartially transmitting film 94, and an opaque film 96. Light 98,incident to reticle 90, is output as transmitted light 100 at aplurality of intensities. To simplify the explanation, the lenstypically used between reticle 90 and a photoresist mask 102 is notshown. Although some exposure systems expose photoresist to the samesize as the reticles, it is typical to use a lens in a reduction systemto create a pattern on the resist that is five, or ten, times smallerthan the reticle pattern. Light transmitted through opaque film 96 issubstantially blocked so as to substantially not effect photoresist 102in a first area 104 of photoresist 102. Light transmitted throughpartially transmitting film 94 has a transmitted intensity of 30%relative to incident light 98 to form a first level 106 in photoresist102. Incident light 98 passing through substrate 92 at a third intensityforms an opening 108 in resist 102.

Alternately, FIG. 13 can be described as reticle 90 transmittingincident light 98 to create a profile pattern with a plurality ofthicknesses 102. Reticle 90 comprising a partially transmitting film 94,partially transmitting film 94 attenuating a predetermined percentage ofincident light to form a photoresist area 106 having a first thickness.Reticle 90 also comprising opaque film 96, opaque film 96 blockingsubstantially all incident light 98 to form photoresist area 104 havinga second thickness greater than the first thickness. Reticle 90comprising a transparent substrate 92, substrate 92 passingsubstantially all incident light to form photoresist area 108 having athird thickness less than the first thickness. In the preferred form ofthe invention the third thickness is substantially zero so that anopening is formed in photoresist film 102.

In yet another alternate explanation, reticle 90 transmits incidentlight 98 to create a profile pattern with a plurality of thicknesses ona light sensitive photoresist film 102, reticle 90 having a transparentsubstrate 92, passing substantially all incident light 98, to form anopening 108 in photoresist film 102, reticle 90 also having opaque film96, blocking substantially all incident light 98 to form a photoresistarea having a second thickness, with reticle 90 comprising partiallytransmitting film 94, attenuating a predetermined percentage of incidentlight 98, to form photoresist area 106 having a first thickness, thefirst thickness being approximately 1/3 the second thickness.

FIG. 14 illustrates the steps of the method of the present invention toform a tri-level reticle. The reticle transmits incident light. It is afeature of the invention that the transmitted light illuminates areas ofa light sensitive surface, each area being illuminated with apredetermined level of transmitted light. Step 120 is providing areticle substrate. Step 122 is depositing at least one film, topartially transmit incident light, over the reticle substrate, thepartially transmitting film diminishing the intensity of light apredetermined percentage in transmission through the partiallytransmitting film, and the substrate passing substantially all lightincident to the substrate. Step 124 is the depositing of an opaque filmover the reticle substrate, the opaque film blocking light so thatsubstantially all incident light is attenuated. Step 126 is etchingselective portions of the opaque film deposited in Step 124, and thepartially transmitting film deposited in step 122, to revealpredetermined areas of reticle substrate and partially transmittingfilm. Step 128 is the product, a reticle to transmit light through thepredetermined areas of reticle substrate, partially transmitting film,and remaining opaque film to produce at least three intensities oflight. It is a feature of the invention that the three intensities oftransmitted light produce a patterned profile of at least twothicknesses in a light sensitive surface, typically with an openingthrough the surface.

It is a feature of the invention that Step 122 includes the depositionof a plurality of partially transmitting films in sequential layers withthe light transmission characteristics of each partially transmittingfilm being cumulative to progressively attenuate incident light passingthrough the partially transmitted films, in which the plurality ofpartially transmitting films have differing etch selectivity, so thatthe etching of adjacent partially transmitting films requires separateetch processes. It is also a feature that Step 126 includes etchingselective portions of predetermined partially transmitting films toreveal underlying partially transmitting film layers, whereby aplurality of partially transmitting film combinations provides aplurality of transmitting light intensities.

It is a feature of the present invention that Step 122 includes thedeposition of a plurality of partially transmitting films, with thecumulative attenuation of the plurality of partially transmitting filmssubstantially replicating the light transmission characteristics of theopaque film, so that Step 124 includes Step 122, since the deposition ofthe plurality of partially transmitting films substantially blocks thetransmission of light.

In the preferred embodiment of the invention Step 122 includes thedeposition of the partially transmitting film in a layer adjacent to,and overlying, the reticle substrate, and Step 124 includes thedeposition of the opaque film in a layer adjacent to, and overlying, thepartially transmitting film, and in which Step 126 includes etchingselected portions of the opaque film so that light is introduced to thepartially transmitting film, without transmission through the opaquefilm, and in which Step 126 includes etching selected portions of thepartially transmitting and opaque films so that light is introduced to aselected portion of the reticle substrate without transmission througheither the opaque film or the partially transmitting film, whereby lightof at least three intensities illuminates the light sensitive surface.

In the preferred form of the invention Step 126 includes the steps offorming a first layer of photoresist, having a first pattern with anopening through the first layer, overlying the opaque film. Step 126also includes the step of etching a predetermined area of the opaquefilm, revealed through the opening in the pattern of the photoresistformed earlier, to reveal a first area of the partially transmittingfilm underlying the opaque film, and etching the first area of thepartially transmitting film to reveal a predetermined area of thereticle substrate underlying the partially transmitting film. Step 126further includes the step of forming ea second layer of photoresist,having a second pattern with an opening through the second layer,overlying the opaque film. Finally, Step 126 includes the step ofetching a predetermined area of the opaque film, revealed through theopening in the second pattern in the photoresist formed above, to reveala second area of the partially transmitting film underlying the opaquefilm, whereby light introduced to the predetermined area of the reticlesubstrate is transmitted with a first intensity, light introduced to thesecond area of the partially transmitting film is transmitted throughthe partially transmitting film and underlying substrate at a secondintensity, and light introduced to the remaining opaque film istransmitted through the opaque film the underlying partiallytransmitting film, and the substrate at a third intensity to illuminatea light sensitive surface.

FIG. 15 illustrates the steps of the method of the present invention toform a multi-level photoresist pattern. Step 140 provides a reticlesubstrate. Step 142 deposits one or more films, to partially transmitincident light, over the reticle substrate, each partially transmittingfilm diminishing the intensity of light a predetermined percentage intransmission through the partially transmitting film, and the reticlesubstrate passing substantially all incident light. Step 144 deposits anopaque film, over the partially transmitting film deposited in step 142,the opaque film blocking light so that substantially all incident lightis attenuated. Step 146 forms a first layer of photoresist film, havinga pattern with a first opening through the photoresist film, over theopaque film deposited in step 144, to reveal a predetermined area ofopaque film. Step 148 etches the predetermined area of opaque film,revealed in step 146, to reveal a predetermined area of partiallytransmitting film.

Step 150 etches the predetermined area of partially transmitting filmrevealed in step 148 to reveal a predetermined area of the reticlesubstrate. Step 152 forms a second layer of photoresist film, having asecond opening through the photoresist film, over the opaque film toreveal a predetermined area of opaque film. Step 154 etches thepredetermined area of opaque film revealed in step 152 to reveal apredetermined area of partially transmitting film. Step 156 exposes alight sensitive photoresist substrate, having a predetermined thickness,to light transmitted through the reticle for a predetermined amount oftime, with light being transmitted through the predetermined area of thereticle substrate revealed in step 150 to expose the first photoresistarea to a first dosage, with light being transmitted through thepredetermined area of the partially transmitting film revealed in step154 to expose a second photoresist area to a second dosage, and withlight being transmitted through the remaining opaque film to expose athird photoresist area to a third dosage. As explained in the discussionof FIG. 8, light dosage is a function of the intensity of thetransmitted light on a surface, and the amount of time the surface isexposed to this light.

Step 158 develops the photoresist substrate exposed in step 156 to forma photoresist profile having an opening in the first photoresist area,the photoresist profile having substantially the photoresistpredetermined thickness in the third photoresist area, and thephotoresist profile having an intermediate thickness, between thepredetermined thickness and zero, in the second photoresist area,whereby light introduced to the reticle, formed by revealingpredetermined areas of reticle substrate and partially transmittingfilm, produces at least three intensities of light. Step 160 is aproduct, a photoresist substrate transformed into a profile of at leasttwo thicknesses and an opening, generally replicating the profile of themulti-level reticle.

FIG. 16 illustrates the steps of the method of the present invention topattern a photoresist film with a plurality of thicknesses from a singleexposure to light. Step 170 provides a light sensitive photoresist filmfrom a single exposure to a light source. Step 172 exposes an area ofthe photoresist to a first intensity of light to form an opening throughthe photoresist film, the first intensity of light being substantiallyunattenuated in transmission from the light source. Step 174 exposes anarea of the photoresist to a second intensity of light to form aphotoresist area having a first thickness, the second intensity of lightbeing partially attenuated in transmission from the light source. Step176 exposes an area of the photoresist to a third intensity of light toform a photoresist area having a second thickness greater than the firstthickness, the third intensity of light being substantially blocked intransmission from the light source. Step 178 is a product, a photoresistfilm with a plurality of thicknesses.

FIG. 17 illustrates the method of the present invention to form aphotoresist profile having two exposure patterns from a single reticle.Step 180 provides a substrate for a photoresist profile. Step 182provides a layer of photoresist having a predetermined thickness on thesubstrate. Step 184 directs light to the photoresist through a reticlehaving a first transmitting intensity to create a first exposurepattern, the reticle having a second transmitting intensity to create asecond exposure pattern. Step 186 develops the photoresist to remove afirst thickness of photoresist, less than the predetermined thickness,in the areas of the first exposure pattern, and to remove a secondthickness of the photoresist in the areas of the second exposurepattern. Step 188 is a product, a profile including areas of photoresisthaving a plurality of different thicknesses.

It is a feature of the invention that step 184 includes directing lightto the photoresist through a reticle having a third transmittingintensity to create a third exposure pattern in the photoresist, andwherein step 186 includes removing a third thickness of the photoresistin the areas of the third exposure pattern, whereby the profile includesareas of photoresist having at least three different thicknesses.

In a preferred embodiment step 184 includes exposing the photoresist tolight at the first transmitting intensity to create the first exposurepattern, and exposing the photoresist to light at the secondtransmitting intensity greater than the first transmitting intensity tocreate the second exposure pattern. In the preferred embodiment thesecond exposure pattern at least partially overlaps the first exposurepattern. In many typical applications the photoresist will be used toform a via with an overlying connecting line. The via is formed by thehole in the resist, or second exposure pattern. The line is formed atthe intermediate level of the resist, or the first exposure pattern. Forthe via and line to electrically communicate the exposure patterns mustoverlap.

The explanation of FIG. 17 is supplemented with notice that the lightused for the second exposure pattern has a larger dosage of photons perunit area than light used for the first exposure pattern, and step 186includes developing the photoresist to remove substantially the fullpredetermined thickness of the photoresist in the areas of the secondexposure pattern and to remove only the first thickness, less than thepredetermined thickness, in the areas of the first exposure pattern. Asexplained in FIG. 8, photoresist is removed by a calculation of dosage.Dosage is a measure of light intensity and time over a given area. Thesame calculation can be performed using photons per unit area.

The explanation of reticles and resist profiles has chiefly focused onthe formation of a bi-level resist profile from a tri-level reticle,however, the present invention is not so limited. The methods detailedabove are applicable to reticles having more than three levels to formresist profile of more than two levels.

FIGS. 18(a) through 18(e) illustrate an alternate embodiment of thepresent invention, a method of forming a multi-level reticle from amulti-level photoresist profile. In FIG. 18(a) a photoresist film 190 isformed over a multi-level reticle 191 comprising an opaque, or chrome,layer 192, a partially transmitting, or half tone (HT) film 194, and aquartz substrate 196. In FIG. 18(b) the resist is removed to two levelsusing an electron beam (e-beam). The e-beam removes resist to anintermediate layer 198, and also totally removes resist to expose anarea 200 of opaque film 192. In FIG. 18(c) reticle 191 is etched so thatopaque film 192 and partially transmitting film 194 are removed beneatharea 200, to reveal an area 202 of substrate 196. In FIG. 18(d) resist190 is partially removed so that an area 204 of opaque film 192 isrevealed. In FIG. 18(e) area 204 of opaque film 192 is etched to revealan area 206 of partially transmitting film 194. In FIG. 18(e) theremaining resist 190 is removed. Reticle 191 of FIG. 18(e) is the sameas reticle 50 of FIG. 5. However, reticle 191 has been formed with onlyone resist development process. Also, the alignment of features isbetter than that possible with the reticle of FIG. 5. Since the reticledoes that have to be aligned with a second resist pattern, as shown inFIG. 4, alignment accuracies of 0.005 microns, between area 206 and 200are achieved. Area 206 corresponds, typically, to the line (trench)interconnects in the intermediate interlevel of a dielectric, and area200 corresponds to vias. The above process describes the formation of atri-level reticle, but it is not so limited. The same process can alsobe applied to form three or more levels of resist over reticle 191 toform a reticle 191 having four or more levels. Modifications andvariations within the scope of the present invention will occur to thoseskilled in the art.

What is claimed is:
 1. A reticle through which at least threeintensities of incident light are passed to define a multi-level profileon a light sensitive photoresist surface, the reticle comprising:a firsttransmission level film producing transmitted light of a firstintensity; a second transmission level film producing transmitted lightof a second, intermediate, intensity greater than the first intensity,and retarding the phase of the transmitted second intensity of lightapproximately 90 degrees; and a third transmission level film producingtransmitted light of a third intensity greater than the secondintensity, whereby the light transmitted through the reticle with thefirst intensity exposes a first photoresist area to a first dosage, thelight transmitted with the second intensity exposes a second photoresistarea to a second dosage, and the light transmitted with the thirdintensity exposes a third photoresist area to a third dosage.
 2. Thereticle as in claim 1 in which said third transmission level film is asubstrate, in which said second transmission level overlies said thirdtransmission level substrate, and in which said first transmission levelfilm overlies said second transmission level film.
 3. The reticle as inclaim 1 in which said first transmission level film is an opaque filmselected from a group consisting of Cr, CrO, and iron oxide, wherebysaid first transmission level film blocks incident light.
 4. The reticleas in claim 1 in which said third transmission level film is selectedfrom a group consisting of quartz, synthetic quartz, and glass, wherebysaid third transmission level film is transparent to pass all incidentlight.
 5. The reticle as in claim 1 in which said first transmissionlevel film has a first transmission level opening, through said firsttransmission level film, to reveal a predetermined area of said secondtransmission level film, and in which said second transmission levelarea has a second transmission level opening, through said secondtransmission level, to reveal a predetermined area of said thirdtransmission level film.
 6. The reticle as in claim 1 in which aplurality of second transmission level films are provided, each of saidplurality of second transmission level films producing transmitted lightat one of a plurality of second intensities greater than the firstintensity and less than the third intensity, whereby light in a range ofsecond intensities is provided.
 7. The reticle as in claim 1 in whichsaid first transmission level film is made up of a plurality of secondtransmission level films, each one of said plurality of secondtransmission level films being a partially transmitting film layer suchthat the intensity characteristics of light transmitted through saidplurality of second level films are cumulative to yield the lighttransmission characteristics of said first transmission level film. 8.The reticle as in claim 7 in which said first transmission level filmincludes a first number of film layers, that is substantially all ofsaid multiple partially transmitting film layers, and in which saidsecond transmission level film includes a second number of said multiplepartially transmitting film layers, fewer than the first number of filmlayers.
 9. The reticle as in claim 1 in which said second transmissionlevel film is selected from the group consisting of indium tin oxide andmolybdenum silicon oxynitride, whereby said second transmission levelfilm attenuates the intensity of transmitted light, a predeterminedpercentage.
 10. The reticle as in claim 1 in which said secondtransmission level film transmits more than approximately 10%, but lessthan approximately 90%, of incident light, whereby the intensity oflight transmitted by said second transmission level film isapproximately midway between said first and third transmission levelfilm intensities, such that the reticle, when directed to a lightsensitive surface, forms at least three distinct intensities on theilluminated areas of photoresist.
 11. A reticle for transmitting atleast three intensities of incident light to create a multi-levelprofile pattern of at least three thicknesses on a light sensitivephotoresist film overlying an integrated circuit interlevel dielectric,the reticle comprising:a partially transmitting film, said partiallytransmitting film retarding the phase of incident light approximately 90degrees and intermediately attenuating incident light to form a firstthickness of photoresist profile; an opaque film, said opaque blockingall incident light to form a second thickness of photoresist profilegreater than the first thickness of photoresist profile; and atransparent substrate, said substrate passing all incident light to forma third thickness of photoresist profile less than the first thicknessof photoresist profile, whereby the attenuation characteristics of saidpartially transmitting film are approximately midway between said opaquefilm and said transparent substrate attenuation characteristics.
 12. Thereticle as in claim 11 in which the third thickness is substantiallyzero so that an opening is formed in the photoresist film.
 13. A reticlefor transmitting incident light to create a profile pattern of at leasttwo thicknesses and an opening, on a light sensitive photoresist filmoverlying an integrated circuit substrate, the reticle having atransparent substrate, passing all incident light, to form an opening inthe photoresist profile, the reticle also having an opaque film,blocking all incident light to form a second thickness in thephotoresist profile, the reticle comprising:a partially transmittingfilm, intermediately attenuating incident light and retarding the phaseof incident light approximately 90 degrees, to form a first thickness inthe photoresist profile, the first thickness of photoresist profilebeing approximately 1/2 the second thickness of photoresist profile,after exposure and development.
 14. A reticle as in claim 10 in whichsaid second transmission level film transmits approximately 30% ofincident light.
 15. A reticle as in claim 11 in which said partiallytransmitting film transmits more than approximately 10%, but less thanapproximately 90% of incident light, whereby the attenuationcharacteristics of said partially transmitting film are approximatelymidway between said opaque film and said transparent substrateattenuation characteristics, such that the reticle, when directed to thelight sensitive surface, forms at least three distinct intensities onthe illuminated areas of photoresist.
 16. A reticle as in claim 15 inwhich said partially transmitting film transmits approximately 30% ofincident light.
 17. A reticle as in claim 13 in which said partiallytransmitting film transmits more than approximately 10%, but less thanapproximately 90% of incident light, whereby the attenuationcharacteristics of said partially transmitting film are approximatelymidway between the opaque film and the transparent substrate attenuationcharacteristics, such that the reticle, when directed to the lightsensitive surface, forms at least three distinct intensities on theilluminated areas of photoresist.
 18. A reticle as in claim 17 in whichsaid partially transmitting film transmits approximately 30% of incidentlight.
 19. A photolithographic reticle to form a dual damascene profilein a photoresist film having a first thickness, the dual damasceneprofile having a via at a third thickness of zero and trench at a secondthickness, intermediate between the first and third thicknesses, from asingle exposure to a light source, the reticle comprising:a partiallytransmitting film, said partially transmitting film retarding the phaseof incident light approximately 90 degrees and intermediatelyattenuating incident light to form the second thickness in thephotoresist profile; an opaque film, said opaque blocking all incidentlight to form the first thickness in the photoresist profile; and atransparent substrate, said substrate passing all incident light to formthe third thickness of photoresist profile, whereby the attenuationcharacteristics of said partially transmitting film are approximatelymidway between said opaque film and said transparent substrateattenuation characteristics.
 20. A reticle as in claim 19 in which saidpartially transmitting film transmits more than approximately 10%, butless than approximately 90% of incident light, whereby the reticle, whendirected to the light sensitive surface, forms at least three distinctintensities on the illuminated areas of photoresist.
 21. A reticle as inclaim 19 in which said partially transmitting film transmitsapproximately 30% of incident light.